1. Field of the Invention
The present invention generally relates to a light source system, in particular, to a light source system and a driving method for light emitting diodes (LEDs).
2. Description of Related Art
Recently, with the vigorous development of semiconductor technology, portable electronic products and flat panel display (FPD) products also rise. Among many types of the FPDs, the liquid crystal display (LCD) immediately becomes the mainstream of various display products due to advantages such as low-voltage operation, radiation-free line scattering, a light weight, and a small volume. Generally, an LCD panel itself does not have the characteristic of light-emission, and thus it is necessary to dispose a backlight module below the LCD panel, so as to provide the required light (backlight) source for the LCD panel.
The conventional backlight module may be roughly divided into two categories: the backlight module formed by cold cathode fluorescent lamps (CCFLs), and the backlight module formed by LEDs. As the LED backlight module is capable of improving the color gamut of the LCD, currently most panel manufacturers replace the CCFL backlight module with the LED backlight module.
The LED backlight module has a plurality of parallel LED strings, and each LED string is formed by a plurality of LEDs connected in series. In practice, each LED string is operated under a direct current (DC) output voltage provided by a boost circuit. Besides, a driver for driving the LED strings generates a plurality of pulsation current signals in response to a setting signal provided by a system terminal, so as to respectively drive the LED strings. Specifically, both the frequency and the duty cycle of each pulsation current signal generated by the driver are in accordance with the frequency and the duty cycle of the setting signal provided by the system terminal.
However, as the frequency of the setting signal provided by the system terminal is limited within 200 Hz to 20 KHz, an output capacitor (Cout) of the boost circuit generates resonance with the print circuit board (PCB) in response to the pulsation of the pulsation current signals generated by the driver, so as to produce acoustic noises. Moreover, if the frequency of the setting signal provided by the system terminal (that is, the frequency of the pulsation current signal) is close to the multiple of the frame rate of the LCD (for example, 240 Hz, 300 Hz, etc.), it is more likely that unnecessary oblique or horizontal stripes (i.e. wave noises) are generated in images displayed by the LCD, which may affect the quality of the frame.